Werk #15450: mk-job: improvements to state file persistence

The mk-job agent plugin saves information about a job's running and completed states in files under $MK_VARDIR/job. These files were written dynamically and this could give rise to situations in which the state file was read while information was being written to it. The information within the <<<job>>> section could therefore be confusing and incomplete. This has been improved by introducing atomic operations: files are now either present and complete, or absent altogether.

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